Abstract [unreadable]3 integrins ([unreadable]v[unreadable]3 and [unreadable]IIb[unreadable]3) play critical roles in tumor metastasis by mediating bone resorption, platelet aggregation, and neo-angiogenesis. The long-term goal of this project is to define the mechanisms by which [unreadable]3 integrins expressed on non-tumor cells contribute to bone metastasis and tumor growth in the bone micro-environment. In the first funding period of this R01, we found that the global [unreadable]3 knockout mouse was protected from tumor-associated bone loss and bone metastasis. Bone marrow transplantation experiments demonstrated that hematopoeitic cells were responsible for metastasis protection, suggesting a role for platelet or osteoclast [unreadable]3 integrins. [unreadable]3 integrins are also expressed on non-hematopoeitic cells like endothelial cells, which can influence tumor biology. We have generated mice with germline-targeted [unreadable]3 integrin gene flanked by loxP sites and deleted the gene in platelets and osteoclast precursors, using Cre transgenic mice. [unreadable]3 integrin function is modulated by P2Y12, an ADP receptor, and the target of the anti- platelet drug, clopidogrel (Plavix). [unreadable]3 integrin ligand binding can also be enhanced by association with CD47, also known as integrin-associated protein. We have data showing that P2Y12 regulates integrin activation and receptor conformation and that P2Y12-/- mice have osteoclast and platelet function defects as seen in the [unreadable]3-/- mice. We have found that CD47-/- have defects in osteoclast function. Based on data that the global [unreadable]3 knockout mouse has decreased bone metastases, but increased tumor associated angiogenesis, and that [unreadable]3 integrin modulating proteins P2Y12 and CD47 play critical roles in osteoclast and platelet function, we hypothesize that 1) [unreadable]3 integrin expression on different tissues and [unreadable]3 integrin regulation by 2) P2Y12 and 3) Integrin associated protein/CD47 will regulate tumor metastasis to bone. Thus, our Specific Aims are: 1. Define role of [unreadable]3 on platelets, osteoclasts/myeloid cells and endothelium during metastasis to bone and tumor growth in the bone micro-environment. 2. Define the role of P2Y12, an upstream activator of [unreadable]3 integrin, on platelets and osteoclasts during skeletal metastasis and tumor osteolysis. 3. Evaluate the role of integrin associated protein (CD47) during skeletal metastasis and tumor osteolysis. Mice harboring tissue-specific deletions of integrin [unreadable]3, and global deletion of CD47 and P2Y12, will be useful tools in assessing the individual roles of [unreadable]3 integrins in tumor cell homing to and growth in bone and may uncover novel molecular targets to more effectively treat skeletal metastasis and pathologic bone loss.